1. In the first 2 cycles of use. Robinson JE, et al. Fertility and Sterility (2007) 87: 329-334
2. In a study of 87 women, 4 or more fertile days were identified in 80% of cycles using actual cycle length.

Sperm in more detail

Each time a man ejaculates, he releases 200 - 400 million sperm. However, all these sperm are not the same, normally a man will produce many different types of sperm: amorphous, coil-tailed, crook necked, double headed (bicephalous), pin-sized, and short-tailed. They also vary in size, and the largest sperm in a single human ejaculate can be almost 8 times larger than the smallest.

This wide variety of sperm types has always been thought to be because sperm are difficult to produce, and maybe also explains why so many are produced. However, if natural selection were taking place, then why should so many sperm be lame, defective, slow or deformed?

An interesting theory to address this was put forward by Dr Robin Baker and Dr Mark Bellis, detailed in their 1996 book 'Sperm Wars'. The 'Kamikaze Sperm Hypothesis' suggests that the sperm's journey to the egg is less of a race and more of a war, in which men's sperm literally do battle with each other.

They proposed that only a small proportion of human sperm are actually intended to function as 'egg-getters' which fertilise eggs; the balance are Kamikaze sperm on a suicide mission to stop the sperm of other males. Their intriguing idea suggests that a man's ejaculate contains three types of sperm, each of which exists for a different purpose: 1) "Fertilisers," the egg-penetration specialists, 2)"Blockers, " to construct copulatory plugs to prevent further sperm from progressing further, 3)"Killers" that hunt down and kill other men's sperm that might be present.

Whilst this is an interesting hypothesis, it is just that, and should be noted that other researchers have been unable to replicate Baker and Bellis's findings. It is, however, an interesting fact that despite millions of sperm entering your body during intercourse, it takes just one to fertilise your egg. If you are trying to get pregnant and would like to know more, click here.